Cancel current command if cache becomes unusable

If the cache suddenly disappears, the current command will now fail.

changelog/unreleased/pull-4166

@@ -0,0 +1,7 @@
+Enhancement: Cancel current command if cache becomes unusable
+
+If the cache directory was removed or ran out of space while restic was
+running, this caused further caching attempts to fail and drastically slow down
+the command execution. Now, the currently running command is canceled instead.
+
+https://github.com/restic/restic/pull/4166